1. Field of the Invention
The present invention relates to a telescopic zoom lens system to be used in binoculars or telescopes.
2. Description of the Related Art
In zoom binoculars or zoom telescopes, it is preferable to secure a wider real field of view to obtain an entire view with backgrounds by setting the binoculars or telescopes at a lower magnification side, and to enlarge an operator""s desired portion of the view by setting the binoculars or telescopes at a higher magnification side. As a zooming system in telescopes and binoculars, an OE-ZOOM lens system, in which a zooming lens group moveable along the optical axis is provided in both the objective optical system and the eyepiece optical system, is generally used. In the OE-ZOOM lens system, the eyepiece optical system which includes a zooming lens group is advantageously constituted by a smaller number of lens elements, e.g., five lens elements, while the overall length of the eyepiece optical system is made constant. However, since zooming is performed in the eyepiece optical system as well, the apparent field of view is varied accordingly. In the OE-ZOOM lens system disclosed in U.S. Pat. No. 5,734,509, the apparent field of view from a lower magnification side to a higher magnification side is about 40xc2x0 to 50xc2x0. In particular, when a combination of the objective optical system and the eyepiece optical systems is considered, the apparent field of view at a lower magnification side (i.e., on the side of a longer focal length of the eyepiece optical system) is narrowed, and further, the real field of view is narrowed.
It is an object of the present invention to provide a telescopic zoom lens system in which the apparent field of view is not varied upon zooming.
In order to achieve the above-mentioned object, there is provided a telescopic zoom lens system in which a minimum magnification thereof is more than 2, and at least a positive objective lens group, and a positive eyepiece lens group are provided in this order from the object. Further, a zooming lens group which is made moveable along the optical axis upon zooming is positioned on the side of the object with respect to the primary image forming plane of the objective optical system. According to this arrangement, the apparent field of view is not varied upon zooming.
The telescopic zoom lens system according to the present invention at least includes, in the following order from the object, a positive first lens group, a positive second lens group and a negative third lens group which constitute the positive objective lens group; and a positive fourth lens group which constitutes the positive eyepiece lens group. The third lens group is the zooming lens group, while the first lens group is a diopter adjusting element and is made moveable along the optical axis in order to adjust the change in the diopter which is occurred when the third lens group is moved.
In the objective lens group, it is common to provide an erecting optical system which is constituted by a plurality of optical elements. In such a case, if the third lens group is a zooming lens group like the above, at least one optical element in the erecting optical system may be made moveable along the optical axis as a diopter adjusting element. For example, a triangle prism can be used as a diopter adjusting element in an erecting optical system.
Moreover, the first lens group is preferably made interchangeable. According to this arrangement, the telescopic zoom lens system can be used in astronomical telescopes.
The telescopic zoom lens system according to the present invention preferably satisfies the following conditions in order to reduce the change in the eye relief and to miniaturize the entire zoom lens system:
1.7 less than |f2/f3| less than 3.5xe2x80x83xe2x80x83(1)
0.4 less than m2wxc3x97m3w less than 0.9xe2x80x83xe2x80x83(2)
wherein
f2 designates the focal length of the second lens group;
f3 designates the focal length of the third lens group;
m2w designates the lateral magnification of the second lens group when the angular magnification of the entire zoom lens system is the minimum; and
m3w designates the lateral magnification of the third lens group when the angular magnification of the entire zoom lens system is the minimum.
The third lens group can be constituted by a single lens element, and in such a case, the following condition is preferably satisfied:
0.7 less than (m2txc3x97m3w)/(m2wxc3x97m3t) less than 1.3xe2x80x83xe2x80x83(3)
wherein
m2t designates the lateral magnification of the second lens group when the angular magnification of the entire zoom lens system is the maximum; and
m3t designates the lateral magnification of the third lens group when the angular magnification of the entire zoom lens system is the maximum.
In the case where the telescopic zoom lens system of the present invention is provided in binoculars, at least a part of either one of the fourth lens groups, which constitute a pair of the eyepiece lens groups, is preferably made to be a diopter-difference adjusting lens element which is moveable along the optical axis.
The present disclosure relates to subject matter contained in Japanese Patent Application No.Hei-11-14591 (filed on Jan. 22, 1999) which is expressly incorporated herein by reference in its entirety.